foreign_packages.sh

#!/bin/bash
if [ ! -x /usr/bin/apt-show-versions ]
then
  echo 'To use this script, please run this command first:'
  echo '  sudo apt install apt-show-versions'
  exit 1
fi
FOREIGN=$(tempfile --prefix=ood_)
/usr/bin/apt-show-versions | grep -Ev ' (uptodate|not installed)$' > $FOREIGN
if [ -e $FOREIGN ]
then
  echo 'Note: No packages / package versions of unknown origin were found' >&2
else
  echo 'Packages / package versions which are installed but of unknown origin:'
  cat $FOREIGN
fi
rm $FOREIGN